Recently, an IGBT (insulated gate bipolar transistor), a diode and the like are used as semiconductor elements used for a power conversion device such as an inverter. The diode is typically used as a back-flow diode and connected antiparallel to the IGBT. Thus, the diode is referred to as free wheeling diode (FWD).
For the characteristics improvement of a power conversion device such as an inverter, the characteristics improvement of the FWD is important along with the characteristics improvement of the IGBT. Important characteristics of the FWD include the on-voltage (voltage drop in the conducting state), switching time (extinction time of the recovery current at turn-off time), and safe operating area at turn-off time (area free from breakdown even under voltage application when the recovery current is flowing). Furthermore, for FWD, it is more desirable that the current/voltage oscillation at turn-off time be smaller. Among them, it is important to expand the safe operating area at turn-off time.